Microorganisms belonging to the genus Rhodococcus are known as microbial catalysts to hydrate nitrites so as to produce corresponding amides or acids. Moreover, the microorganisms also exert a great variety of properties in producing enzymes involved in degradation of PCB (polychlorinated biphenyl) or the like or desulfurization of crude oil, as well as producing biosurfactants which are used, for example, for effluent treatment, so that they are known to be industrially very useful microorganisms.
In addition, microorganisms belonging to the species of Rhodococcus rhodochrous are known to have extremely highly efficient nitrile hydration activity, so that they are industrially utilized as catalysts for producing acrylamide through biotechnology.
The term “ATCC” where it appears in the specification below refers to the American Type Culture Collection® or ATCC®.
Under such circumstances, the development of host vector systems of the genus Rhodococcus has been expected, and some such systems have been developed. Examples of industrially useful plasmid vectors of bacteria of the genus Rhodococcus include plasmid pRC001 carried by Rhodococcus rhodochrous ATCC4276, pRC002 carried by Rhodococcus rhodochrous ATCC14349, pRC003 carried by Rhodococcus rhodochrous ATCC14348, plasmid pRC004 (see Japanese Patent No. 2983602) carried by Rhodococcus rhodochrous IFO3338, and composite plasmid vectors pK1, pK2, pK3 and pK4 (see JP Patent Publication (Kokai) No. 5-64589 A (1993)) comprising a plasmid DNA region capable of duplicating and replicating within the cells of Escherichia coli and a DNA region containing a drug resistance gene and pRC020 (see JP Patent Publication (Kokai) No. 9-28379 A (1997)) carried by Rhodococcus erythropolis IFO12320. When a useful gene is introduced into a plasmid vector and then expressed by bacteria of the genus Rhodococcus as a host, it is known that the expression level of this useful gene is almost proportional to the number of copies of the plasmid vector. However, the number of copies of these plasmids within the cells of the bacteria of the genus Rhodococcus is around 2 to 6, which is not always sufficient to cause high expression of a foreign gene.